Pictures posted to social media purportedly of the attack showed smoke rising from the area.

Karachi, Pakistan's largest city and a financial hub, was for years rife with political, sectarian and ethnic militancy.

A crackdown in the city by security forces in recent years has brought a lull in violence, but scattered attacks still take place.

China, one of Pakistan's closest allies, has poured billions into the South Asian country in recent years as part of a massive infrastructure project that seeks to connect its western province Xinjiang with the Arabian Sea.